You could also try give the little ones an "escape" area. Somewhere that they can fit, but the bigger ones can't. Like a small door between pens, etc. Or even just obstacles for them to run around and behind. The older ones get tired of that game quickly. lol
I have several cutouts in my largest Serama pen, and have about 4 different age ranges in there. The little ones dart back and forth, around branches, etc, and the bigger ones give up trying to catch them after a while.
Splitting them up is good, but if you plan to reintroduce them later, it "might' be easier to just let them sort it out. Please don't think I'm cruel. I feel terrible that they actually killed one. But usually it doesn't get that bad. But they do have to establish pecking order.